Conventionally, a frequency division duplex (FDD) scheme for dividing an uplink and a downlink based on a frequency and a time division duplex (TDD) scheme for dividing an uplink and a downlink based on a time are known as duplex schemes in a wireless communication system. In the FDD scheme, an uplink signal and a downlink signal are transmitted and received at different frequencies at the same time. On the other hand, in the TDD scheme, the uplink signal and the downlink signal are transmitted and received at different times at the same frequency.
In a wireless communication system using a TDD scheme, since transmission and reception are alternately performed at the same frequency in a wireless base station, transmission ON periods and transmission OFF periods are alternately present in a transmission signal that is transmitted from the wireless base station, as illustrated in FIG. 5.
If a power in the transmission ON period is referred to as a transmission ON power, and a power in the transmission OFF period is referred to as a transmission OFF power, there is a target value of each of the transmission ON power and the transmission OFF power, and evaluation of each target value is performed at the time of evaluation of the wireless base station. In particular, the target value of the transmission OFF power is close to −114 dBm/MHz that is a thermal noise level and is, for example, a very low power of about −107 dBm/MHz. In a case in which the transmission OFF power is measured, it is necessary to use a measurement device of which the noise floor level is sufficiently lower than the transmission OFF power level.
When the transmission OFF power is measured, components such as a cable or a switch are required between the wireless base station and the measurement device. Accordingly, the transmission OFF power level is reduced by such components. Therefore, a configuration of amplifying a signal from the wireless base station before the signal is input to the measurement device as illustrated in FIG. 6 is considered.
That is, a signal measurement system 50 illustrated in FIG. 6 includes a DUT 51 as a wireless base station which is a measurement target, a signal measurement device that measures a transmission OFF power of a signal output by the DUT 51, and a transmission power measurement system 53 provided between the DUT 51 and the signal measurement device 52. The transmission power measurement system 53 includes a low noise amplifier (LNA) 53a that amplifies the transmission OFF power. A trigger signal for synchronizing a frame is output from the DUT 51, and the signal measurement device 52 measures a transmission OFF power level based on the trigger signal. A spectrum analyzer that achieves reduction of a noise floor level (for example, see Patent Document 1) can be suitably used as the signal measurement device 52.